The present invention relates to a protection circuit that protects a circuit from an overvoltage.
When a voltage higher than the withstand voltage is accidentally applied to circuit components used in semiconductor integrated circuits, they could function improperly, and even be permanently destroyed in the worst case. Therefore, such semiconductor integrated circuits are provided with protection circuits to protect them from an overvoltage. Patent literature 1 (Japanese Unexamined Patent Application Publication No. 2010-111213) discloses such a protection circuit.
FIG. 10 shows an overvoltage protection circuit disclosed in Patent literature 1. In FIG. 10, an overvoltage protection circuit B is provided to prevent an overvoltage from being applied to a circuit A to be protected. In a normal operation, a voltage applied from a battery power supply 1 is applied to the circuit A to be protected through a switch 2 and a P-channel MOS transistor 3.
If the battery power supply 1 generates an overvoltage, it exceeds the breakdown voltage of a zener diode ZD and a breakdown current thereby flows through the zener diode ZD. As a result, a voltage at the base of an NPN transistor 5 is raised by resistors R6 and R7, and the impedance between the collector and emitter of the NPN transistor 5 is thereby lowered. Then, a voltage at the base of a PNP transistor 4 is lowered by resistors R4 and R5, and the impedance between the collector and emitter of the PNP transistor 4 is thereby lowered.
As a result, the P-channel MOS transistor 3 becomes an Off state. At this point, a voltage is applied to the circuit A to be protected through a surge protection resistor R1. A surge generated in the battery power supply 1 is consumed by the surge protection resistor R1 and the circuit to be protected located behind the surge protection resistor R1 is thereby protected from the overvoltage.
Meanwhile, Patent literature 2 (Japanese Unexamined Utility Model Application Publication No. 62-107527) discloses a surge absorber circuit that prevents a surge current caused in a load side from flowing backward to external circuits.